An explosive Channel 4 documentary is set to reveal details of how Prince Philip’s sister met and admired Adolf Hitler, whom she described as a ‘charming and seemingly modest man’.

Three of his sisters were married to senior Nazi officers – one a colonel in the SS – and when Philip attended a family funeral in Germany in 1937 he was confronted by crowds giving the Heil Hitler salute. The documentary includes previously unseen extracts from a private memoir in which one sister talks of her early admiration for Hitler.

Meanwhile the footage showing the Queen performing a Nazi salute as a child could have been accidentally leaked by Buckingham Palace, it has been reported.

The Duke of Edinburgh, now 94, served bravely in the Royal Navy during the Second World War and his family’s history is no secret. But the Palace will not welcome a fresh focus on his sister’s links to the Third Reich while controversy continues over the footage of the Queen.

The grainy film was shot in 1933, months after Hitler came to power and six years before the start of the Second World War. It shows the then seven-year-old Princess Elizabeth playing with a dog on the lawn at Balmoral with her sister Margaret, before raising her arm in a Nazi salute.

Her mother, the future Queen Mother, and her uncle Edward – later Edward VIII – are also shown in the pose. Her uncle appears to be showing them how to perform the Nazi salute, which even then was recognised internationally as a sign of obedience to Hitler.

Royal aides are investigating how the 17-second black and white film clip came to be published by The Sun, and Buckingham Palace has not ruled out legal action.

The footage was sold to a tabloid newspaper for a four-figure sum, the Mail has learnt.

It is understood that the 17-second cine film clip was copied by a researcher with ‘access to the royal archives’ several years ago.

The anonymous individual chose not to make it public, however, until a few weeks ago, when they approached The Sun.

It is unclear as to whether the seller obtained it from the British Film Institute, where much of the Royal Family’s historical home cine footage is kept, or from the royal archives at Windsor Castle.

It emerged last night that the edited clip was allegedly used in an exhibition at the Palace last year.

It may have been inadvertently released to a documentary maker after a flurry of requests for the unseen royal home movies, according to the Daily Telegraph.

Buckingham Palace is considering taking legal action over the footage.

The Palace has launched an inquiry into how the 17-second black and white film came into the hands of The Sun newspaper.

Meanwhile the Channel 4 documentary is set to reveal how animosity to the Queen’s marriage to Philip was fuelled by fears about his family’s links to Nazi Germany. In the previously unpublished memoir, his older sister Sophie – a godmother to Prince Edward – wrote of entertaining both Hitler and his henchman Hermann Goering before they rose to power.

She said she and her husband were ‘impressed’ by Hitler and his plans for Germany’s future, although she wrote they had ‘fundamentally’ changed their view some years later. Sophie joined the Nazi Women’s Auxiliary and was photographed at Goering’s wedding, on the top table with Hitler, and her husband Prince Christoph von Hessen was head of the SS in the Air Ministry.

The couple named one of their sons Karl Adolf in Hitler’s honour after meeting the Nazi leader in 1931 or 1932, before he became the German Chancellor.

Philip’s sister said Goering had been ‘very enthusiastic’ about the Nazi party and its leader, and had urged her and her husband to meet Hitler in person. She noted in her memoir, written in old age: ‘As Germany was going through hard times and there was a lot of poverty and general dissatisfaction everywhere, we were interested to hear about the great improvements his party was planning to do.

‘As Goering was insistent we should meet Hitler personally, we decided to ask him to lunch at our flat… I have to say here, that, although Chri [her husband Prince Christoph] and I changed our political view fundamentally some years later, we were impressed by this charming and seemingly modest man, and by his plans to change and improve the situation in Germany.’

Sophie’s memoir was never published and the Channel 4 documentary Prince Philip: The Plot To Make A King – due to be aired on July 30 – is the first time it has been made public by her family.

Three of Philip’s four sisters married German officers and when he travelled to a family funeral in Darmstadt in 1937 he was photographed walking through ranks of troops and crowds giving the Nazi salute. His sisters were excluded from receiving invitations to his wedding to the future Queen in 1947 because of their links to Nazi Germany.

But Sophie, whose husband Prince Christoph was killed in 1943 and who remarried to Prince George William of Hanover, was made a godmother to Prince Edward in 1964 and was a frequent visitor to the Royal Family.